Vibration generators are used in a great many technical fields for various applications. For example, vibration generators are used in material conveying machinery to encourage flow or settling of powders, granules and thick liquids or slurries. Vibration generators are also used for inducing movement of parts in vibratory bowl feeders and other material conveying machinery. Other applications of vibration generators include material screening or sieving, sanding, material finishing and frictional welding machinery.
Orbital vibrators, which produce a circular or elliptical motion, generally without rotation or angular velocity, are useful for many of these applications. One particular application that benefits from the use of an orbital vibrator is for agitating the distributor cone of a combination weigher. Commonly, orbital vibrators operate by rotating an eccentrically weighted flywheel using an air motor or an electric motor. Eccentric weighted air motors provide good orbital agitation, are adjustable in speed and amplitude and are quick starting and stopping, which are important attributes in this application. However, the speed of rotation and amplitude are highly interdependent, making it very difficult to independently adjust the frequency and amplitude of the orbital motion. In addition, because they are driven by compressed air, they are expensive to run, and they are considered unsanitary in some applications, such as handling of pharmaceuticals and food products. Eccentric weighted electric motors have the advantage that they do not require compressed air to operate. However, speed control requires expensive control circuitry and the motor coasts considerably when turned off, giving poor on-off control. This can be improved by adding a brake to the motor, but this adds considerable expense and adds wear and maintenance parts to the mechanism. Electromagnetic vibration generators are also used in material conveying equipment, such as vibratory bowl feeders, however these are generally rotational or reciprocating vibrators and are not suitable for applications where orbital vibration is required.
Electromagnetic orbital vibrators have been devised for use in other fields. For example, U.S. Pat. Nos. 5,160,393, 5,277,744 and 5,378,951, granted to Snyder, describe a friction welder with an electromagnetic orbital vibrator. The orbital vibrator has three electromagnets arranged around an armature of magnetically attracted material. The three electromagnets are powered by a variable frequency three-phase alternating current source to induce an orbital motion in the armature. A mode selector switch is described, which allows the friction welder to be operated alternatively in an orbital vibratory mode and in a reciprocating vibratory mode. Since this electromagnetic orbital vibrator is configured specifically for use in a friction welder, it is not readily adaptable for use in material conveying equipment and many other applications. Furthermore, the requirement for an additional variable frequency three-phase alternating current source to operate the vibrator mechanism is a significant limitation in the use of this apparatus.